Power amplifiers are used in transmitters to rebroadcast, at high power levels, amplitude modulation (AM) signals, frequency modulation (FM) signals, and quadrature amplitude modulation (QAM) signals. As is known, information is carried in the amplitudes of AM and QAM signals. Because of this, transmitters must accurately rebroadcast these signals, thus the power amplifiers within the transmitters must have high fidelity, i.e. linearly rebroadcast the signals they receive.
To maintain linear performance, the power elements (usually transistors) are operated at levels much below their rated output power levels. Thus, to achieve high output power levels, many power amplifiers include cascaded elements, such that the amplified output of one element is the input of the next element. In this manner, a small change in a first stage typically produces a large change in the final stage output. For this reason, the power amplifier utilizes a feedback control loop, which is regulated by a power control circuit, to maintain output power levels.
However, when changes exceed the limits of the feedback control loop, damage can occur to the power elements (transistors). Once damage has occurred, excessive feedback signal levels produce power amplifier instability which results in the destruction of the power amplifier.
Therefore a need exists for a method and apparatus that provides protection of power amplifiers from excessive operating power levels.